This invention relates in general to electrophotography and more specifically, to an improved electrophotographic imaging member having improved charge generation layers and process for using the imaging member.
In the art of electrophotography, an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer in either a flexible belt form or a rigid drum configuration. For most multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the back side of the substrate support, opposite to the side of the electrically active layers, to render the desired photoreceptor flatness. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate charge generating (photogenerating) and charge transport layers. The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. The photogenerating layer utilized in multilayered photoreceptors include, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Inorganic or organic photoconductive material may be formed as a continuous, homogeneous photogenerating layer. Many suitable photogenerating materials known in the art can be utilized, if desired.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated, duplicating and printing systems employed flexible photoreceptor belts, operating at very high speeds, have also placed stringent mechanical requirements and narrow operating limits as well on photoreceptors. For example, the layers of many modern multilayered photoreceptor belt must be highly flexible, adhere well to each other, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles.
A typical prior art multilayered flexible photoreceptor configuration comprising an adhesive interface layer between the hole blocking layer and the adjacent photogenerating layer to improve adhesion or to act as an electrical barrier layer, is disclosed, for example, in U.S. Pat. No. 4,780,385. Typical adhesive interface layers disclosed in U.S. Pat. No. 4,780,385 include film-forming polymers such as polyester, polyvinylbutyral, polyvinylpyrolidone, polyurethane, polycarbonates polymethylmethacrylate, mixtures thereof, and the like. Specific polyester adhesive materials are disclosed, for example in U.S. Pat. No. 4,786,570 where linear saturated copolyesters consisting of alternating monomer units of ethylene glycol and four randomly sequenced diacids and copolyesters of diacids and diols where the diacid is selected from the group consisting of terephthalic acid, isophthalic acid, adipic acid, azelaic acid, and mixtures thereof and the diol is selected from the group consisting of ethylene glycol, 2,2-dimethyl propane diol and mixtures thereof. The entire disclosure of U.S. Pat. No. 4,786,570 is incorporated herein by reference.
An encouraging advance in electrophotographic imaging which has emerged in recent years is the successful fabrication of a flexible imaging member which exhibits excellent capacitive charging characteristic, outstanding photosensitivity, low electrical potential dark decay, and long term electrical cyclic stability. This imaging member employed in belt form usually comprises a substrate, a conductive layer, a solution coated hole blocking layer, a solution coated adhesive layer, a thin charge generating layer comprising a sublimation deposited perylene or phthalocyanine organic pigment or a dispersion of one of these pigments in a selected binder resin, a solution coated charge transport layer, a solution coated anti-curl layer, and an optional overcoating layer.
Multi-layered photoreceptors containing charge generating layers, comprising either vacuum sublimation deposited pure organic pigment or an organic pigment dispersion of perylene or phthalocyanine in a resin binder, have frequently been found to have undesirable characteristics such as forming charge deficient spots which are visible in the final hard copy print. Photoreceptors containing perylene pigments in the charge generating layers, particularly benzimidazole perylene dispersion charge generating layers, have a spectral sensitivity of up to 720 nanometers, are highly compatible with exposure systems utilizing visible laser diodes, exhibit low dark decay electrical characteristic and reduced background/residual voltages. These characteristics are superior to photoreceptor counterparts containing a trigonal selenium dispersion in the charge generating layer. Unfortunately, these multi-layered benzimidazole perylene photoreceptors have also been found to develop a serious charge deficient spots problem, particularly the dispersion of perylene pigment in the matrix of a bisphenol Z type polycarbonate film forming binder. The expression "charge deficient spots" as employed herein is defined as localized areas of dark decay that appear as toner deficient spots when using charged area development, e.g. appearance of small white spots having an average size of between about 0.2 and about 0.3 millimeter on a black toner background on an imaged hard copy. In discharged area development systems, the charge deficient spots appear in the output copies as small black toner spots on a white background. Moreover, multi-layered benzimidazole perylene photoreceptors have also been noted to yield low adhesion bond strength at the contacting surfaces between the charge generating layer and the adhesive interface layer, causing undesirable premature photoreceptor layer delamination during photoreceptor image cycling in copiers, duplicators and printers. In a customer service environment, premature photoreceptor layer delamination requires costly and frequent photoreceptor belt replacement by skilled technical representatives.
Typically, flexible photoreceptor belts are fabricated by depositing the various layers of photoactive coatings onto long webs which are thereafter cut into sheets. The opposite ends of each photoreceptor sheet are overlapped and ultrasonically welded together to form an imaging belt. In order to increase throughput during the web coating operation, the webs to be coated have a width of twice the width of a final belt. After coating, the web is slit lengthwise and thereafter transversely cut into predetermined length to form photoreceptor sheets of precise dimensions that are eventually welded into belts. When multi-layered photoreceptors containing perylene pigment dispersion in the charge generating layer are slit lengthwise during the belt fabrication process, it has been found that some of the photoreceptor delaminates and becomes unusable. In the fabricated belt form, photoreceptor layer delamination at the welded seam, due to stress concentration development at the double thickness overlap area during dynamic fatigue photoreceptor belt bending/flexing over the machine belt support rollers, diminishes the practical application value of the belt. All of the above deficiencies, implicated by the low layer adhesion bond strength, hinder slitting of a photoreceptor web through the charge generating layer without encountering edge delamination. Slitting is used to transversely cut webs into sheets for welding into belts and also to longitudinally slice double wide coated photoreceptor webs into multiple narrower charge generating layers.
In general, photoconductive pigment loadings of 80 percent by volume in a binder resin or a mixed resins binder are highly desirable in the photogenerating layer to provide excellent photosensitivity. However, these dispersions are highly unstable to extrusion coating conditions, resulting in numerous coating defects that generate a large number of unacceptable material that must be scrapped when using extrusion coating of a dispersion of pigment in organic solution of polymeric binder. More stable dispersions can be obtained by reducing the pigment loading to 30-40 percent by volume, but in most cases the resulting "diluted" photogenerating layer could not provide adequate photosensitivity. Also, the dispersions of higher pigment loadings generally provided a generator layer with poor to adequate adhesion to either the underlying ground plane or adhesive layer, or the overlying transport layer when polyvinylbutyral binders are utilized in the charge generating layer. Many of these organic dispersions are quite unstable with respect to pigment agglomeration, resulting in dispersion settling and the formation of dark streaks and spots of pigment during the coating process. Normally, the polymeric binders which produce the best (most stable, therefore most manufacturable) dispersion suffer from deficiencies either in xerographic or mechanical properties, while the least stable dispersions provided the best possible mechanical and xerographic properties. The best compromise of manufacturability and xerographic/mechanical performance is obtained by use of a photogenerating layer containing benzimidazole perylene pigment dispersed in a bisphenol Z type polycarbonate film forming binder. However, when a polyester adhesive layer is employed in a photoreceptor in combination with a photogenerating layer containing benzimidazole perylene pigment dispersed in a bisphenol A type or a bisphenol Z type polycarbonate film forming binder, poor adhesion between the charge generator layer and the adhesive layer can cause spontaneous photoreceptor delaminate during certain slitting operations, during fabrication, or during extensive photoreceptor belt cycling over small diameter machine belt support rollers.
In addition, when a multilayered belt imaging member containing benzimidazole perylene pigment dispersed in the bisphenol Z polycarbonate film forming binder in the charge generating layer is fabricated by ultrasonic welding the opposite ends of an imaging sheet together, delamination is encountered when attempts are made to grind away some of the weld splash material. Removal of the weld splash material is of particular important, because it allows the elimination of seams which form flaps during electrophotographic imaging and cleaning processes of belt function that causes the initiation of toner particles trapping and thereafter release them as unwanted dirts over the imaging belt surface to result in copy black spot print defects. Also, the inability to grind, buff, or polish a welded seam causes reduced cleaning blade life as well as seam interference with toner image ultrasonic transfer assist subsystems.
In U.S. Pat. No. 5,322,755 a layered photoconductive imaging member is disclosed comprising a supporting substrate, a photogenerator layer comprising perylene photoconductive pigments dispersed in a resin binder mixture comprising at least two polymers, and a charge transport layer. The resin binder can be, for example, a mixture of polyvinylcarbazole and polycarbonate homopolymer or a mixture of polyvinylcarbazole, polyvinylbutyral and polycarbonate homopolymer or a mixture of polyvinylcarbazole and polyvinylbutyral or a mixture of polyvinylcarbazole and a polyester. Although improvement in photosensitivity and adhesion are achieved, charge deficient spots print defects can still be a problem.
Thus, there is a continuing need for improved photoreceptors that exhibit freedom from charge deficient spots and are more resistant to layer delamination during slitting, grinding, buffing, polishing, and dynamic belt image cycling.